1. Field of the Invention
The present invention relates to burners and, more particularly, to low NOx emission burners having staged air and staged fuel capabilities.
2. Brief Description of the Prior Art
Low NOx burners are known in the art. For example, U.S. Pat. Nos. 5,180,300 and 4,983,118 both disclose low NOx regenerative burners. Likewise, U.S. Pat. No. 4,732,093 to Hansen et al. discloses a method and apparatus for burning fuel in an annular nozzle burner. However, there exists a need for a burner that further reduces NOx generation.
The present invention provides an air and fuel staged burner that reduces NOx generation. One embodiment of a burner according the present invention generally includes a main burner body defining an internal cavity, an air connection fluidly connected to the internal cavity, and a combustion tunnel. A distribution tee may be fluidly connected to the internal cavity defined by the main burner body and a burner nozzle may be positioned in the interior cavity of the main burner body. The burner nozzle may define a primary air orifice, an annulus, and a fuel orifice. The air connection may be configured to receive supply air and divide the supply air into primary air and secondary air, where the ratio of primary air to secondary air is approximately in the range of 40/60 to 70/30 respectively, with a 50/50 ratio being preferred. The primary air preferably flows through the primary air orifice at a rate of approximately 300-400 feet/second (91-122 meters/second).
The main burner body generally extends longitudinally about an imaginary burner centerline, and the primary air orifice is preferably oriented to form a convergent angle as measured from the imaginary burner centerline, such as an angle of approximately 30-60xc2x0 as measured from the imaginary burner centerline. Alternatively, the primary air orifice may be oriented to produce a swirl pattern of primary air in the combustion tunnel, where the swirl is approximately less than or equal to 0.7 times an internal diameter of the combustion tunnel.
The burner may also include a secondary air conduit fluidly connected to the distribution tee, the secondary air conduit having a secondary air jet fluidly connected to a secondary combustion zone. The main burner body generally extends longitudinally about an imaginary burner centerline and the secondary air jet is oriented substantially parallel to the imaginary burner centerline. Alternatively, the main burner body may extend longitudinally about the imaginary burner centerline with the secondary air jet oriented at an angle convergent with the imaginary burner centerline. The secondary air exits the secondary air jet at a velocity of approximately 150-400 feet/second (46-122 meters/second).
A fuel connector is configured to receive a supply fuel and divide the supply fuel into a primary fuel and a secondary fuel. The split ratio of primary fuel to secondary fuel split ratio is approximately in the range of 20/80 to 40/60 respectively, with a split ratio of 22/78 being preferred. A primary fuel path and a secondary fuel path may also be included, with the primary fuel path fluidly connected to the annulus, the secondary fuel path fluidly connected to the fuel orifice, and the primary fuel path and the secondary fuel path fluidly connected to each other. The primary fuel may exit the annulus defined by the burner nozzle at a velocity approximately less than 100 feet/second (30 meters/second). The secondary fuel may exit the fuel orifice defined by the burner nozzle at a velocity approximately greater than 350 feet/second. The fuel orifice and the fuel annulus may lie in the same plane, substantially perpendicular to an imaginary burner centerline and the distribution tee may be positioned adjacent to the internal cavity of the main burner body and opposite the combustion tunnel (52).
One method of decreasing NOx emissions in a burner having a main burner body defining a combustion tunnel may include the steps of flowing supply air into the main burner body, dividing the supply air into primary air and secondary air, flowing the primary air into the combustion tunnel at a given velocity, flowing primary fuel into the combustion tunnel at a velocity lower than the velocity of the primary air, flowing secondary fuel into the combustion tunnel at a velocity higher than the velocity of the primary fuel, flowing secondary air into a secondary combustion zone by a secondary air jet at a velocity higher than the velocity of the primary fuel, and igniting the primary fuel, the secondary fuel, and primary air in the combustion tunnel to form products of combustion. Additional steps may include exhausting products of combustion into the secondary combustion zone and drawing products of combustion into the combustion tunnel and into the secondary air jet.
The device and method according to the present invention helps to reduce burner NOx emissions.
These and other features and advantages of the present invention will be clarified in the description of the preferred embodiment taken together with the attached drawings in which like reference numerals represent like elements throughout.